In present day commerce, many materials are transported in over-the-road vehicles such as vans and trucks. It frequently is the case that materials transported must be prevented from varying in temperature as the temperature ambient to the vehicle varies. Many materials such as foodstuffs may not be allowed to become too warm in summer, while others must not be allowed to freeze in winter transport. It is advantageous to provide vehicles with apparatus to supply either heating or cooling, in accordance with seasonal demands, so that the same vehicles can be used year round.
It is, of course, known to make the walls of such vehicles of thermally insulative materials, but it is unavoidable to have the vehicle doors open for appreciable intervals during a delivery schedule, so that means for the addition or removal of heat must be made a part of the vehicle.